Such an insulating barrier is for example used in a non-volatile memory (NVM) device, in which the first conductive region is for example a floating gate. In such memory devices it is desired that charge which is put on the floating gate can remain there for a long period of time, (for example 10 years) which period is referred to as the retention time. On the other hand it is desired that the charge can be removed from the floating gate by tunnelling in a very short time (for example in the order of milli- or microseconds), which time is referred to as the erase time, or transferred to the floating gate by tunnelling in an even shorter time (for example in the order of microseconds), which time is referred to as the write time. In order to achieve such short write/erase times, a suitable tunnelling voltage is applied over the insulating barrier, which is above the maximum read-disturbance voltage (i.e. the maximum voltage at which no significant undesired tunnelling occurs). This suitable tunnelling voltage is strongly dependent on the dielectric material in which the barrier is constructed and by its thickness.
Conventionally, the insulating barrier is constructed as a single layer of a dielectric material. The material and the thickness of the layer are chosen in function of a desired compromise between, on the one hand, obtaining a maximum read disturbance voltage as high as possible and, on the other hand, making it possible to write or erase the floating gate by applying a voltage as low as possible. Such a single-layer insulating barrier has an energy band diagram as shown in FIGS. 2 and 3, FIG. 2 showing the diagram in absence of a voltage applied over the barrier and FIG. 3 showing the diagram upon applying the tunnelling voltage. FIG. 3 clearly shows that the tunnelling voltage of the single-layer insulating barrier is undesirably high.
An insulating barrier which requires a lower voltage for tunnelling is for example known from U.S. Pat. No. 6,121,654. This document discloses a non-volatile memory device having an insulating barrier which is constructed as a three-layered structure. A first layer and a third layer of the insulating barrier are constructed in a low-barrier material and a second layer which is interposed between the first and third layers is constructed in a high-barrier material. As a result, the insulating barrier has a stepped energy band diagram with a lower level over the first and third layers and a higher level over the second layer (see FIGS. 5 and 6). The low- and high-barrier materials have substantially the same dielectric constant. As a consequence, the energy band diagram of the barrier during tunnelling has substantially the same inclination over the three layers. By applying the low-barrier material between the first conductive region, i.e. the floating gate, and the high-barrier material of the second layer, it is achieved that the peak of the energy band diagram during tunnelling is reduced with respect to that of a single-layer insulating barrier. As a result, the tunnelling voltage of the insulating barrier is reduced with respect to that of the single-layer insulating barrier.
However, the insulating barrier described in U.S. Pat. No. 6,121,654 has the disadvantage that such a barrier cannot easily be constructed, as there are only few suitable material combinations, and that all of these combinations have practical problems. U.S. Pat. No. 6,121,654 describes only one possible combination, namely Si3N4 as low-barrier material and AlN as high-barrier material. Depositing an AlN-layer is mostly done by epitaxial growth, wherein the ordered structure of the underlying layer is used as a template to assemble a high quality AlN layer. However, Si3N4 does not have the required ordered structure and therefore the structure of the AlN layer that is deposited on a Si3N4 layer will have defects that can compromise the long term data retention. Hence AlN is not a suitable material for constructing such an insulating barrier.